The present invention pertains generally to the display of polygonal video images on a raster-scan display screen and more particularly to a method for generating the boundaries that are used to form the polygonal areas displayed.
The generation of line segments and, therefore, polygonal areas bounded by a number of such line segments, is easily accomplished by the vector generation technique. This technique draws straight lines by moving a cathode ray tube (CRT) beam continuously from a starting point to an ending point. By drawing a number of such straight lines end-to-end via this technique, a polygonal area is formed on the CRT. However, vector generation techniques cannot distinguish one polygonal area from another other than by size and shape.
Raster-type displays, that is, those utilizing an image-forming beam traversing a display screen in a plurality of horizontal lines, can add a further dimension to the display of polygonal areas. By appropriate control of the luminence and chroma information, the areas within the boundaries of any polygonal figure can be further distinguished from other polygonal figures by color or shading.
However, it has heretofore been the practice of generating polygonal areas on a raster-type display screen by storing a binary representation of the entire area in a random access memory (RAM). This binary information is read from the RAM in synchronism with the active scan of the display beam to display the polygonal area. If a number of such polygonal areas are to be generated, the memory space required can become exorbitant. Further, if the polygonal areas are to change in size or shape, the requirement for memory space increases accordingly, as does the circuitry required to handle these binary representations. A final disadvantage of this technique is that the handling of the required data is slow, leading to severe timing problems when the many different polygonal representations are to be displayed.